I'm not sure exactly how much I have lowered my car (mk2 Driver +20mm higher over a gti). Around 40-50mm roughly. When I fitted my coilovers I left the height at how they came in the box. Which happened to be about 80-90mm, didn't look v good to me. I kept it like that for a few hours and a giggle but it's just not worth it when you felt how harsh and crap the ride was + you couldn't full lock, go over speed bumps properly, carry passengers + girls just get pissed off with it etc etc. I think for practicality no more than 40mm for a gti.
It comes down to personal choice really... Mines about 100mm on FK Highsports... ride's firm, not hard and it handles beautifully.

The only thing is that the front splitter rubs as does the exhaust occasionally depending on the size of humps. It doesn't bother me in the slightest if i have to slow down as it makes up for it on the handling. TBH it is best to lower it till the wishbones are straight as too low can put the camber out but then again I have had no problems at all.
It your car son... do what you wanna do... personally i'd slam it but subtle lowering is just as effective.
